A number of physical and chemical methods have, in the past, been utilized to generate or recover anhydrous hydrogen fluoride gas from various aqueous hydrofluoric acid solutions. The existing methods suffer many disadvantages including low yield and low purity of the desired final product, high complexity in the design and operation of the process apparatus and difficulty in the disposal and/or neutralization of the waste or residue from which the hydrogen fluoride gas evolves. Many of the existing methods for the recovery of hydrogen fluoride are suitable only for batch mode processing which, in and of itself, has disadvantages when compared to the preferred continuous process mode.
The recovery of anhydrous hydrogen fluoride gas from aqueous solutions often is carried out as a step necessary to render economical some other process which results in the generation of large quantities of impure hydrofluoric acid. For example, U.S. Pat. No. 2,939,766 (Churchill, June 7, 1960) relates to the recovery of hydrogen fluoride from a mixture which consists essentially of an alkali metal bifluoride and hydrogen fluoride - water azeotrope. The mixture results during the preparation of fluorobenzene by the diazotization of aniline and subsequent decomposition of the benzenediazonium fluoride in the presence of excess hydrogen fluoride. In the Churchill process, which is carried out in the batch mode, the above described mixture is cooled to 0.degree. C., admixed with sulfur trioxide and then heated to distill off hydrogen fluoride. The process is a batch process, however, uses a relatively large amount of sulfur trioxide and is relatively energy intensive. The large quantity of sulfur trioxide may lead to contamination of the final hydrogen fluoride product, and the initial cooling step requires excess time and energy.
U.S. Pat. No. 4,120,939 (Ehlig, Oct. 17, 1978) discloses a continuous process for the production of hydrogen fluoride by dropping small particles of a metal fluoride, such as calcium fluoride (fluorspar), through a reaction zone countercurrent to a gas stream containing sulfur trioxide, sulfuric acid and water vapor which is introduced at the bottom of the reactor zone. Optionally, hydrogen fluoride or another gas inert to the reaction (such as air) also is introduced at the bottom of the reactor zone.